6th Annual Summer Symposium-Less Familiar Elements
Spectrophotometric Determination of Selenium and Tellurium in Concentrated Sulfuric Acid STEPHEN E. WIBERLEY, LEWIS G. BASSETT, ARTHUR 41. BURRILL', AND HELEN LYNG2 Rensselaer Polytechnic Institute, Troy, A'. Y . The need for determining small amounts of tellurium in magnesium alloys led to the development of a spectrophotometric method. Selenium was included in the investigation because of its similar chemical nature. Tellurium or selenium is precipitated from solution by reduction, and dissolved-in concentrated sulfuric acid. The absorbance of the solution is measured at 350 mp for selenium and at 520 mp for tellurium. Using a 1-cm. cell, 50 y of tellurium or 200 y of selenium per 50 ml. of sulfuric acid solution can be determined. Tellurium in magnesium alloys has been successfully determined by this method. Selenium can be determined in the presence of equal amounts of tellurium, but tellurium cannot be determined in the presence of equal amounts of selenium.
A
QUAXTITATIVE colorimetric method for the determination of small quantities of tellurium and possibly selenium was desired for the analysis of magnesium alloys. Feigl ( 3 ) has reported a qualitative method for the detection of selenium or tellurium based on the color of these elements in concentrated sulfuric acid. According to Feigl, a green or red solution results because of the formation of loose addition compounds between selenium or tellurium and the anhydride of sulfuric acid according to the following equations
+ H2S04 Te + H2S04 Se
-f
-+
Se SO3
+ H?O
Te So3
+ HzO
These compounds are decomposed into their components by water, and red selenium or black tellurium precipitates. Boitsova and Butkov (I)measured the absorption spectrum of tellurium dissolved in concentrated sulfuric acid with a quartz spectrograph. They reported an absorption maximum a t 5180 A. and a minimum a t 3960 A. Moles ( 5 ) made cryoscopic measurements on solutions of selenium and tellurium in concentrated and fuming sulfuric acid. The molecular weights he obtained varied considerably with the type of acid used. He believed the complex of selenium to be SeS03 or a polymer (SeS03),. hloles also reported that the solubility of selenium in concentrated sulfuric acid is greater for the red amorphous variety than for the gray metallic selenium. Auerbach ( 1 ) reported, however, that "metallic" selenium dissolves in fuming sulfuric acid as Se and tellurium as Te. To the authors' knowledge no mention has been made in the literature of the quantitative determination of selenium and tellurium based on the color of their complexes in concentrated sulfuric acid. This investigation waa undertaken to see whether such an approach was feasible.
hydrochloric acid. The tellurium is precipitated as a finely divided solid with sulfur dioxide, washed with xater followed by alcohol, and then dried a t 100" C. Sulfuric acid (98%), reagent grade Sulfurous acid (6'33, sulfur dioxide minimum), reagent grade Stannous chloride solution, 115 grams of stannous chloride dissolved in 170 ml. of 12N hydrochloric acid. The resulting solution is diluted to 1 liter and stored over tin. Beckman Model B spectrophotometer, All measurements are made in 1-em. cells. PROCEDURE
Selenium. Concentrated sulfuric acid a t a temperature of 175" j=5" C. is added to the selenium in a 150-ml. beaker. The solution is maintained a t 175' C. for 15 minutes, and then allowed to cool. After the solution has been transferred to a 50or 100-ml. borosilicate glass volumetric flask and diluted to the mark N-ith concentrated sulfuric acid, the absorbance is measured a t 350 mp with a suitable spectrophotometer. Tellurium. The tellurium is dissolved by a procedure similar to that described for the selenium, except that the temperature of the sulfuric acid should be 100" =t5' C., the time of heating should be 10 minutes, and the absorbance should be measured a t 520 mg.
o,m
0.250
t
TELLURIUM
I 6
REAGENTS AND EQUIPMENT
Selenium (red powder of estimated purity better than 99.9%), obtained from Eimer and Amend. Tellurium (estimated purity better than 99.0%), obtained from the Raritan Copper Works. To obtain powdered tellurium, flakes of the tellurium are dissolved in 6 N nitric acid. The solution is evaporated to a small volume and then diluted with 3 N I 2
Present address, Procter and Gamble Co., Cincinnati, Ohio. Present address. E. I. du P o n t de Nemours & Co., Inc., Wilmington, Del.
8
C 0 N C E N T R AT I ON ( m p / I O 0 ml.)
Figure 1. Calibration Curves for Determination of Selenium and Tellurium
Calibration Curves. Stock solutions containing known amounts of selenium and tellurium were prepared according to the procedures just described. Suitable aliquots of these stock solutions were added to 50- or 100-ml. borosilicate glass volumetric flasks and diluted to the mark with concentrated sulfuric
1586
V O L U M E 2 5 , NO. 11, N O V E M B E R 1 9 5 3 acid. The absorbancies of these solutions were measured against concentrated sulfuric acid in the reference cell and plotted against the concentration. Typical calibration curves for selenium and tellurium are shown in Figure 1. The straight-line relationship between absorbance and concentration s h o w that these solutions obey Beer's law. Analysis of Samples. -4 sample of the unknown material containing from 2 to 10 mg. of selenium or 0.5 to 4 mg. of tellurium is dissolved with heating in 25 ml. (or more if necessary) of 3N hydrochloric acid. Approximately 2 ml. of 611; nitric acid is then added to dissolve the selenium or tellurium.
1587 at 420 mp, the Ivave length finally selected for ineasurement was 350 mp. Thi- wave length shows less interference from the tellurium and greater sensitivity than the one a t 420 mp. In addition, the peak at 420 mp varies more critically with the time of heating and the temperature of the sulfuric acid used in dissolving the selenium. Values below 350 mp are not satisfactory, as sulfuric acid begins to absorb.
Table I.
Analysis of Magnesium Alloys Sample Xumber 487
486
485
% Tellurium Colorimetric Gravimetric
+
0.134 5p.p.t. 0.123 4=3Op.p.t.
0.093 i 2 0 y . p . t . 0.087+35p.ri.t.
0.096 f 16p.p.t. 0.089 & 2 8 p . p . t .
The absorption spectrum of a solution containing 0.04 mg. of tellurium per ml. of concentrated sulfuric acid was also determined from 320 to 700 mp. The results are plotted in Figure 2 for comparison with the selenium. In the case of tellurium a sharp maximum occurs a t 520 mp, and this wave length was selected for measurement of the tellurium complex.
T E L L UAI U Y
0.400
Figure 2. Absorption Spectra of Selenium and Tellurium in Concentrated Sulfuric Acid
The solution is evaporated to a small volume and then diluted again with 3N hydrochloric acid. The selenium or tellurium is then precipitated with 5 ml. of stannous chloride solution. The precipitate is filtered on a small sintered-glass filter funnel with suction, washed with water and finally with alcohol, and then dried a t 100" C. Approximately 35 ml. of hot concentrated sulfuric acid is added to a small beaker containing the sintered-glass funnel. The temperature of the acid should be 175' C. for the selenium and 100" C . for the tellurium, and should be maintained for 15 minutes for the selenium and 10 minutes for the tellurium. After cooling, the solution is transferred to a 50-ml. volumetric flask and diluted to the mark with concentrated sulfuric acid. The absorbance of the solution is measured a t 350 mp for the selenium and a t 520 mp for the tellurium, and the concentration obtained from the standard calibration curve. Three samples of magnesium alloys containing 5 % zinc and small amounts of tellurium were analyzed by this procedure. For comparison, these samples were also analyzed by a standard gravimetric procedure ( 4 ) using sulfurous acid as the reducing agent. Three determinations were made on each sample and the precision is reported as the average deviation. The results are shown in Table I. I n every case slightly higher results were obtained by the colorimetric method. This may have been caused by incomplete precipitation of the tellurium in the gravimetric method, since the stannous chloride is a more efficient reducing agent than sulfurous acid. Stannous chloride was not used in the gravimetric procedure because it coprecipitates with the tellurium ( 4 ) . The colorimetric procedure required smaller samples (2 grams as compared to 15 grams) and yielded better precision (14 as compared to 30 parts per thousand). DISCUSSION OF VARIABLES
Absorption Spectra. A solution containing 0.156 mg. of selenium per ml. of concentrated sulfuric acid was prepared according to the recommended procedure. The absorption spectrum was measured between 320 and 700 mp. The results obtained are plotted in Figure 2 . Although a slight peak occurs
0.200
t ?
0.100
0 000,
S
TIME ( min.)
W 0
2 0400-
e
-
10
IS
SELENIUM 175' C
rn
o.oooo
I S
IO
IS
I
1
20
TIME (min.)
Figure 3. Effect of Temperature and Time of Heating on Absorbancies of Selenium and Tellurium Complexes
Temperature and Time of Heating. To determine the optimum temperature and the time of heating for the analysis of selenium, 10-mg. samples of selenium were treated with hot concentrated acid maintained a t known teniperatures for known periods of time. The temperature \vas varied between 100" and 235' C. and the time between 2.5 and 20 minutes. After cooling, the samples were diluted t o 100 ml. in volumetric flasks. The results are shown in Figure 3. .kt temperatures below 175" C. the results were erratic, as the samples did not completely dissolve. dbove 175' C. the samples dissolved completely but the colored complex was unstable and very dependent upon the time of heating. For these reasons, the temperature selected was 175' C. and the time of heating selected was 15 minutes. A similar set of experiments was carried out for tellurium.
ANALYTICAL CHEMISTRY
1588 For the determination of tellurium the optimum temperature wm 100' C. and the opt,imum time of heating was 10 minutes (See Figure 3). Absorption of Water Vapor. Because sulfuric acid is a strong dehydrating agent, the effect of adding water to the selenium and tellurium complexes in concentrated sulfuric acid wm studied. The amounts of water added varied from 0 to 10% of the total volume of the solution. In the case of ~eleriium.the peak a t 420 mp disappears upon the addition of water. The absorbance a t 350 or 375 n i remains ~ unchanged. With larger amounts of water, selenium precipitates. I n the cme of tellurium the absorbance a t 520 mp decreases slightly upon the addition of water as the tellurium precipitates. I n an analysis, if the volumetric flasks are kept s t o p p e d , the absorption of water vapor will not cause noticeable errors. Stability of Solutions. Both the selenium and tellurium complexes in concentrated sulfuric acid are very stable on standing. Over a period of 24 hours there is no change in absorbance, :md after several weeks the change in absorbance is less than 10%. Mixtures of Selenium and Tellurium. As there is such a wide difference in the temperatures necessary to dissolve the .selenium and tellurium, several niixtures of approximatel)- equal
amounts of selenium and tellurium were prepared, and then analyzed by dissolving the mixture a t either 175" or 100" C. In the mixtures analyzed for selenium by heating at a temperature of 175" C., the tellurium was converted to a colorless form, and the results for selenium were approximately 5% high. In the case of the tellurium very erratic results were olitained on the mixtures dissolved at 100" C. Even at 100" C. soine of the selenium dissolves and destroys the color of the tellurium complex. Apparently the rate of disappearance of the tellurium complex is dependent upon the amount of selenium prewit in the sulfuric acid solution. LITERATURE CITED
(1) Auerbach, R., 2. phusik. Chem., 121, 337 (19%). ( 2 ) Boitsova, Z. V., and Butkov, IC. V., Physik. 2. Socoj,jrtrc~iio~i, 12,458 (1937). (3) Feigl, F., "Chemistry of Specific, Selective and Sensitive Reactions," p. 38, Xew York, Academic Press, 1949. (4) Hillebrand, W. F., and Lundell, G. E. F., "Applied Inorganic Analysis," p. 260, New York, John Wiley 8; Sons, 1929. (5) Moles, E., . 4 ~ 1 1 e s soc. esptafi. fk.u q t b h . , 13, 134 (1915). RECEIVED for review June 2G, 1 9 3 .
Accepted October 1 6 , 19%
6th Annual Summer Symposium-Less Familiar Elements
Determination of Impurities in Germanium and Silicon C. L. LUKE AND M4RY E. CAMPBELL Re11 Telephone Laboratories, Inc., M u r r a y Hill, il'. J . As a result of the current interest in the use of semiconductors in electronics i t has been necessary to develop quantitative chemical methods for the determination of metal impurities in the semiconductor materials employed. Photometric methods have been developed for the determination of 0.1 to 1 p.p.m. of arsenic, phosphorus, antimony, and copper in germanium and germanium dioxide. In addition, a method for the determination of 1 to 10 p.p.m. of arsenic in silicon metal has been developed. The methods are designed to be used for quality control of raw materials and as an aid in research and manufacturing processes.
I
N CONJUNCTION with the intensive research on semiconductors that is in progress in these laboratories it has been necessary to develop quantitative chemical methods for the determination of metal impurities in the semiconductor materials used. The authors' analytical program calls for the development of methods of analysis for the Group V metals (arsenic, antimony, and phosphorus), for the Group I11 metals (aluminum, gallium, indium, and boron), and for copper, in germanium dioxide and in germanium arid silicon metal. To date, methods for the determination of 0.1 to 1 p.p.m. of arsenic, phosphorus. antimony, and copper in germanium and germanium dioxide have been completed. I n addition, a method for the determination of,1 to 10 p.p.m. of arsenic in silicon metal has been developed. In trace analysis it is usually necessary to isolate the impurity in question before attempting its determination. Fortunately, germanium can be easily removed by distillation as chloride without loss of most of the metal impurities mentioned. Arsenic and boron must be separated by other means. In view of the small amounts of metal impurities present in the samples to be analyzed, very sensitive methods are required. Because spectrophotometry is one of the analyst's most sensitive tools, it appeared that this might be a good approach to the problem. This has proved to be true. By choosing the most sen-
sitive photometric methods, by using as large a sample and as long a light path as possible, and by purifying reagents, extremely high sensitivity has been obtained. In order to keep the blank due to the glassware to the minimum, Vycor No. 7900 glassware was used wherever posuible. ,2 Beckman DU spectrophotometer with a tungsten lamp and 5-em. Core.; absorption cells \vas used in the photometric work. 111 all the methods developed, precautions have been taken to prevent interference of other metals that may be encountered. In some instances this has necessitated the use of a number of separations and other operations. If any or all of the interfering metal ions can be shown to be absent, by qualitative spectrochemical analysis or by other means, it is obvious that the methods can be greatly simplified. The following metals were used in the specificity tests of the methods developed: aluminum, antimony, arsenic, barium, beryllium, bismuth, boron, cadmium, calcium, cerium, chromium, cobalt, copper, gallium, germanium, gold, hafnium, indium, iridium, iron, lanthanum, lead, lithium, magnesium, manganese, mercury, molybdenum, neodymium, nickel, niobium, osmium, palladium, phosphorus, platinum, potassium, rhenium, rhodium, ruthenium, samarium, scandium, selenium, silicon, silver, sodium, strontium, tantalum, tellurium, thallium, thorium, tin, titanium, tungsten, uranium, vanadium, yttrium, zinc, and zirconium.
